Ball screw drive having an axially supported threaded spindle

ABSTRACT

A ball screw ( 7, 24 ), having a threaded spindle ( 8, 28 ) which is mounted in a rolling fashion by balls ( 9, 27 ) on a threaded nut ( 10, 26 ) and which is supported on an axial bearing ( 13, 25 ), wherein the threaded spindle ( 8, 28 ) is axially supported with its first bearing surface ( 18, 23 ), which is convexly shaped with a radius of curvature (R 1 ), on a preferably conically designed second bearing surface ( 19, 22 ), wherein the first bearing surface ( 18, 23 ) and the second bearing surface ( 19, 22 ) make contact with one another along a contact path ( 37 ), wherein a quotient formed from the ratio of the radius of curvature (R 1 ) to the radius (R 2 ) of the contact path ( 37 ) assumes values of between 1.2 and 2.4 inclusive.

BACKGROUND

The present invention relates to a ball screw. Ball screws convert rotational movements into translatory movements. The present invention also relates in particular to an actuating device for actuating a brake, in particular parking brake for a motor vehicle.

EP 1058795 B1, for example, discloses an actuating device for a parking brake of a motor vehicle, in which actuating device a ball screw is provided.

The threaded spindle, which is driven by an electric motor and which is comprised of a plurality of spindle parts, effects a relative axial displacement between the threaded nut and the threaded spindle, wherein the threaded nut, in its feed direction, exerts a pressure force on a friction pad of a disk brake. This pressure force is absorbed via an axial bearing on the housing, on which axial bearing the threaded spindle is supported. On account of the axial pressure forces exerted, elastic deformations may occur in the housing, which elastic deformations may cause tilting between the threaded spindle and the threaded nut or tilting between the threaded spindle and the axial bearing. Such tilting may impair the functionality of the ball screw.

SUMMARY

It was an object of the present invention to specify a ball screw whose functionality is ensured even under high loads. According to the invention, said object is achieved by means of the ball screw according to claim 1. Correct functionality of the ball screw is realized in that the threaded spindle is supported with its first bearing surface, which is convexly shaped with a radius of curvature, on a preferably conically designed second bearing surface of the axial bearing, wherein the first bearing surface and the second bearing surface make contact with one another along a preferably annular contact path, and wherein furthermore a quotient formed from the ratio of the radius of curvature to the radius of the preferably annular contact path exhibits values of between 1.2 and 2.4 inclusive.

A circle central point with said radius of curvature of the first bearing surface should lie in the axial direction as close as possible to the circle central point of the annular contact path in order to permit the easiest possible tilting. However, if—as is preferable according to the invention—said two circle central points lie on the spindle axis, a minimum spacing between said two circle central points should be maintained; the smaller the spacing, the smaller the surface area available for transmitting axial forces; the larger said spacing becomes, the larger also the surface area for the transmission of axial forces. However, this advantage is opposed by the disadvantage of reduced tilting mobility. Furthermore, for design reasons, a minimum radius for the contact path is necessary if the threaded spindle is guided through the axial bearing; in this case, the radius of the contact path must be greater than the radius of the spindle portion guided through.

The invention has recognized that, taking into account design considerations, the technical problems in conjunction with the mentioned bending moments, the ratios provided according to the invention of the radius of curvature to the radius of the contact path, eliminates the stated disadvantages.

The design according to the invention of the two bearing surfaces permits tilting between the threaded spindle and the axial bearing without the possibility of wear-promoting misalignment or a revolving point load in the region of the axial bearing occurring. A revolving point load would be encountered if no tilting were permitted in the event of a deformation on the housing. A bending moment introduced into the axial bearing would then duly act statically; on account of the rotating spindle with the likewise rotating axial bearing, the location of the introduced bending moment would pass with each revolution, such that a revolving point load in relation to the axial bearing would be realized. The invention avoids this disadvantage: this is because, on account of the design according to the invention of the first and second bearing surfaces of the axial bearing, tilting between the first and second bearing surfaces is permitted, such that during a rotation, a wobbling movement takes place between the first and second bearing surfaces, without undesired bending moments being introduced into the axial bearing.

With the preferred conical design of the second bearing surface, an annular contact path between the two bearing surfaces is obtained without problems, wherein with a slight tilting of the threaded spindle relative to the housing, the contact path can easily drift and deviate from its ideal circular path. The invention prevents the transmission of bending moments into the axial bearing in the event of tilting. Said contact path is also the line of greatest contact pressure which can be determined by the method of Hertzian stress. Without tilting, said contact path has a purely circular shape.

In refinements according to the invention, the threaded spindle may be of single-piece or else multi-piece design, wherein the threaded spindle may have a radially tapered spindle portion which may be designed for being driven by a motor. A shoulder may be formed at the transition from the radially enlarged spindle portion which interacts with the threaded nut to the tapered spindle portion, on which shoulder the first convex bearing surface is formed.

As viewed in longitudinal section through the threaded spindle, the first bearing surface is convexly arched with a radius of curvature. Said convex arching is accordingly evident, in the case of ball screws according to the invention, in longitudinal section through the ball screw. The circle central point preferably lies with the radius of curvature of the first bearing surface on the spindle axis of the threaded spindle. Likewise situated on said spindle axis is the circle central point of the annular contact path. The two central points are however axially spaced apart from one another. The contact path and its central point lie on a plane arranged perpendicular to the spindle axis.

According to the invention, the quotient formed from the ratio of the radius of curvature of the first bearing surface to the diameter of the contact path is specified between the values of 1.2 and 2.4 inclusive. Within said values, the mentioned tilting movements can be permitted without impairment of the functionality of the ball screw. The values according to the invention relate to the load-free starting situation in which contact is duly produced between the first and second bearing surfaces along the contact path, but there is no tilting and no significant axial loading.

Both bearing surfaces rotate with the driven threaded spindle.

It has been found that ball screws according to the invention are particularly suitable as actuating devices in a brake of a motor vehicle, in particular in so-called immobilizing brakes or parking brakes. In such arrangements, as a result of high pressure forces, elastic deformations may arise in the region of the brake caliper, as a result of which tilting between the threaded spindle and the threaded nut, or tilting between the threaded spindle and the axial bearing supported on the housing, may occur. Said tilting angle may be approximately 0.5°. It has been found that, for such a tilting angle, the first convex bearing surface and the second, preferably conical bearing surface are designed such that the quotient formed from the ratio of the radius of curvature to the radius of the contact path assumes values of between 1.4 and 1.6 inclusive. Here, tilting occurs without an introduction of bending moments into the axial bearing. The radius of the contact path is approximately between 7 mm and 10 mm.

If ball screws according to the invention are used in the parking brake mentioned further above, considerable pressure forces can be transmitted in the axial bearing when the brake is activated. In order that tilting as a result of elastic deformation on the housing is permitted even under high axial pressure forces, one refinement according to the invention provides that at least one of the two first and second bearing surfaces is provided with depressions for receiving lubricant. In such refinements according to the invention, there is accordingly a lubricating film along the contact path, such that tilting is possible.

The axial bearing may have a support disk which is connected to the threaded spindle in a positively locking manner in the rotational directions, the second bearing surface being formed on that side of said support disk which faces toward the threaded nut. Said support disk is preferably arranged, so as to be capable of performing a wobbling motion, on the threaded spindle in order to permit the described tilting between the axial bearing and the threaded spindle, without undesired moments being introduced into the axial bearing. Said support disk may then be provided, on the side facing away from the threaded nut, with an axial bearing surface which is formed either directly on the support disk or else on a bearing disk which adjoins the support disk. In this way, the threaded spindle is rotatable relative to a housing but is supported on said housing in the axial direction.

It has accordingly proven to be a problem in brake devices that bending of the brake caliper occurs as a result of the considerable actuating forces when the brake pads are pressed against the brake disk. Said brake caliper bends “open”, that is to say, in effect, the ball screw which is fixedly mounted on the brake caliper is supported against the brake disk and, at a sufficiently high axial contact pressure, bends the brake caliper open. This results in an axial offset, together with angle errors, of the nut with respect to spindle and any axial and radial bearings. This leads to a highly uneven load distribution in the bearings by means of which the ball screw is rotatably mounted on the brake caliper, which can lead to premature failure.

Refinements according to the invention in actuating devices for parking brakes may provide that the threaded nut is connected to a piston which has the brake pad, which piston is mounted on the threaded nut so as to be tiltable relative to the threaded nut, and/or that the threaded spindle is mounted on the brake caliper so as to be tiltable relative to the brake caliper.

In said parking brake, there is a certain degree of relative mobility of different components which are loaded as a result of the operation of the ball screw during braking, which mobility permits a change in the relative position of the components with respect to one another, whereby any axial offset or a bearing load can be at least partially compensated. It is thus provided, according to one alternative of the invention, that the threaded nut is connected to a piston or said piston is mounted directly on the threaded nut, not rigidly but such that said piston can be tilted about the spindle longitudinal axis (which coincides with the nut longitudinal axis). That is to say, the piston can tilt or pivot slightly relative to the threaded nut in every direction. As a result, during the spreading of the brake caliper, the spindle axis and therefore also the nut axis is duly likewise tilted slightly, but said tilting does not directly also act on the coupled piston. In fact, said piston can tilt slightly relative to the nut, as a result of which load compensation is obtained in said region. A movable system is thus realized here.

Said tiltability of the threaded nut may be provided in addition to the described tilting mobility according to the invention in the axial bearing.

If the brake caliper is now spread apart, the caliper bending can be compensated as a result of an at least partial compensation of the position change by means of a slight tilting of the spindle in the movable mounting on the brake caliper. That is to say that, in said region too, a compensating movement facility is realized which leads to a relief of load from the bearing.

In a refinement of the invention, in the case of tiltable mounting of the piston on the nut, it may be provided that, to realize said tiltable mounting, a first conical guide surface is provided on the nut and a second conical guide surface is provided on the piston, which guide surfaces bear against one another, wherein at least one of the guide surfaces is crowned. The nut and the piston make contact with one another only in the region of the guide surfaces which are both basically conical, such that a centering action is obtained. At least one of the guide surfaces, however, has a slight crowning, which makes it possible for the piston to tilt slightly (for example by 0.5° or less during operation) relative to the nut. It is also conceivable for both guide surfaces to be crowned. The piston itself is expediently of hollow cylindrical design, with the threaded spindle being held in the piston preferably over the entire length of its threaded section. This leads to a substantially closed system, and the nut is consequently encapsulated, in effect, in the piston.

BRIEF DESCRIPTION OF THE DRAWINGS

Two exemplary embodiments of the invention are illustrated in the drawing and will be described in more detail below. In the drawing:

FIG. 1 shows a diagrammatic, sectional illustration of a brake device having a ball screw according to the invention in the unloaded state,

FIG. 2 shows an enlarged detail view of the region II from FIG. 1,

FIG. 3 shows an enlarged detail view of the region III from FIG. 1, and

FIG. 4 shows the brake device from FIG. 1 in the loaded state with elements tilted relative to one another,

FIG. 5 shows, in section, a further brake device having a ball screw according to the invention,

FIG. 6 shows the ball screw from FIG. 5, and

FIG. 7 shows an enlarged detail from FIG. 6,

FIG. 8 shows individual parts of the ball screw from FIG. 6,

FIG. 9 shows a further individual part of the ball screw from FIG. 6,

FIG. 10 shows the ball screw according to the invention in a partially cut-away illustration, and

FIG. 11 shows the ball screw according to the invention from FIG. 10 in cross section along the section line XI-XI.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a brake device 1 according to the invention of the type which may be implemented as a parking brake or immobilizing brake in a motor vehicle, for example. The brake device 1 comprises a brake disk 2, which is connected in a known way to the wheel, and a brake caliper 3 of substantially C-shaped cross section, which fits over the brake disk 2. Accommodated in said brake caliper are two brake pads 4, 5, which are positioned on both sides of the brake disk 2 arranged between them and, for the purpose of braking, bear firmly against the latter, clamping the brake disk between them. FIG. 1 shows the release position, that is to say when the brake disk 2 is not clamped and the brake disk 2 can rotate freely between the two brake pads 4, 5, even though these are resting directly against the brake disk for the sake of the illustration. In actual fact, there is a minimal gap between the brake disk 2 and the brake pads 4, 5, allowing free rotation in the release position.

FIG. 1 furthermore shows a ball screw 7 according to the invention, which is accommodated in a portion 6 of the brake caliper 3 that may be formed in the manner of a housing and which comprises a threaded spindle 8, on which a threaded nut 10 runs in a manner guided by balls 9, the balls 9 circulating continuously and being constantly returned by means of at least one ball return element 11. The spindle 8 is connected to a drive motor (not shown in any more detail here), which is preferably arranged in the region of the outside of the housing-like portion 6 and the output shaft of which is at an angle of 90°, for example, to the threaded spindle 8. The output shaft of said drive motor is coupled to the threaded spindle 8 by way of a cardan joint, which allows the threaded spindle 8 to be motor-driven. The threaded spindle 8 is furthermore rotatably mounted in a fixed position on the brake caliper 3 by means of a radial bearing 12 and an axial bearing 13, in the present case in the form of a needle-roller bearing.

The threaded nut 10, for its part, is coupled to a piston 14, and the said piston rests on the front end edge of the threaded nut 10, that is to say is supported there. The movable brake pad 5 is arranged on the piston 14. If the drive motor (not shown in any more detail) is now activated, by actuation of a suitable actuating element on the vehicle, in order to actuate the brake device and hence to fix the brake disk 2, the threaded spindle 8 is moved by the drive motor and rotates, with the result that the threaded nut 10 travels along the threaded spindle 8, being guided by the balls 9 in the process, that is to say moves to the left in FIG. 1. During this process, the piston 14 seated on the end face of the threaded nut 10, and together with it the brake pad 5, is pushed to the left, with the result that it is brought firmly into contact with the brake disk, which is supported against the other brake pad 4, whereby the said brake disk is fixed between the two brake pads 4, 5.

FIG. 2 shows on an enlarged scale a detail view of the seating region of the piston 14 on the threaded nut 10. The piston 14 has a conical guide surface 15, opposite which is a second guide surface 16 on the end of the threaded nut 10, the said second guide surface likewise being conical in terms of its basic shape but having a crowned or convex external form. This means that there is no extensive contact here but only linear bearing of guide surface 15 on guide surface 16. The effect is that the piston 14 is seated in a movable fashion on the nut 10, that is to say guide surface 15 can move on guide surface 16 owing to the linear support. The piston 14 can therefore tilt relative to the threaded nut 10 and a movable bearing arrangement is achieved, with lubrication by means of a suitable lubricant to reduce friction.

As FIG. 3 shows in an enlarged detail view, a bearing arrangement which is likewise movable is achieved in the region of support of the threaded spindle 8 on the brake caliper 3. As explained, the threaded spindle 8 is supported on the wall 17 of the brake caliper, on the one hand radially by means of the radial bearing 12 and, on the other hand, by means of the axial bearing 13. This axial bearing comprises a first bearing disk 18 (housing disk), which is arranged in a fixed position on the wall 17, and a second bearing disk 19 (shaft disk), which runs on the first bearing disk 18 by way of needle rolling bodies 20. Bearing disk 19 has an axial projection 21, which has a conical second bearing surface 22 that, like guide surface 16 in the arrangement for supporting the piston 14 on the threaded nut 10, has a crowned convex surface with a basic shape that is preferably substantially conical.

The threaded spindle 8, for its part, has a first, convex bearing surface 23. It is therefore evident in this case also that a movable bearing arrangement is achieved since, here too, the first bearing surface 23 rests on the second bearing surface 22 only along a line but not over an area. The effect is that the threaded spindle 8 can tilt slightly relative to the positionally fixed axial bearing 13, specifically relative to the positionally fixed bearing disk 19, lubrication likewise being provided. This tilting is made possible by the fact that the threaded spindle 8 is likewise accommodated with a certain play in the radial bearing 12, or the radial bearing, a plastic plain bearing for example, allows a certain tilting. During operation, when the caliper expands owing to the forces that are acting, the tilt angle is in a range of significantly <0.5° per movable bearing location and, as a result, the plain bearing 12 is not subjected to significant loads.

Of course, it is possible with both bearing locations to implement the crowning on the respective other guide surface or to make both guide surfaces crowned.

Thus, in the brake device 1 according to the invention, two movable bearing locations are implemented, namely in the region of the seating of the piston 14 on the nut 10 on the one hand, and in the region of the seating of the threaded spindle 8 on the axial bearing 13 on the other hand. The effect is then that tilting of the relevant axes, which is present in known brake devices and results in high bearing loads that can lead to premature bearing failure, can be compensated to a large extent, thus making it possible to significantly reduce bearing loads.

In the unloaded position shown in FIG. 1, the three longitudinal axes of the threaded spindle 8, the brake caliper 3 or, more specifically, the preferably cylindrical housing-like portion 6, and the piston 14 coincide and are denoted in this Figure as a common axis with the letter A.

If the motor (not shown) is now used to activate the threaded spindle 8 and, by means of the latter, the piston 14 and with it the brake pad 5 is pressed against the brake disk 2, the brake caliper 3 is expanded or spread apart to a greater or lesser extent, depending on the contact force, as shown in FIG. 4. As can be seen, the brake caliper 3 expands and, on the one hand, a slight gap 24 is formed in the region of brake caliper contact with the first brake pad 4, and, as can also clearly be seen, portion 6 of the brake caliper 3 adopts an angled position relative to the piston 14. At this point, it should be pointed out that FIG. 4 shows a significantly exaggerated expansion and tilting of components compared with that which occurs in reality, this being for the sake of illustration.

By virtue of the two separate instances of mobility or movable bearing arrangements that are implemented, however, this severe angular offset can be effectively split up and the load acting on the axial bearing can be significantly reduced. This is because, on the one hand, the tilting of the brake caliper 3, that is to say its spreading apart, has the effect that the piston 14 tilts slightly relative to the nut 10, this being obtained by means of the movable seating of the piston 14 on the nut 10 via the guide surfaces 15, 16, as shown in detail in FIG. 2. In the same way, there is slight tilting of the seating of the threaded spindle 8 on the axial bearing 13 or bearing disk 19 by virtue of the movable bearing arrangement implemented there, as shown in FIG. 3. Here too, there is therefore an albeit slight relative movement or tilting movement. That is to say that the piston 14, the threaded nut 10, the threaded spindle 8, and the axial bearing 13 or bearing disk 19 consequently adjust relative to one another in pairs under the effect of load and consequently there is splitting and hence, at the same time, a local reduction of the individual tilt angles. The movement of the axial bearing 13 relative to the threaded spindle 8 also has the effect that the threaded spindle 8 moves or tilts relative to the radial bearing 12, as is likewise illustrated in FIG. 4. While all the longitudinal axes coincide in FIG. 1 as described, there is now an axial offset owing to the expansion of the brake caliper, but this is significantly less owing to the instances of mobility achieved than it would be with a rigid bearing arrangement. As can be seen, the individual axes A₁ of the brake caliper 3, A₂ of the ball screw 7 or threaded spindle 8, and A₃ of the piston 14 no longer coincide, but the respective axial offset is nevertheless relatively small. The maximum skewing or tilting of about 0.5° of the brake caliper axis relative to the normal to the brake disk which occurs in actual operating conditions can be well compensated by the decoupling of the elements which is provided for by the invention, that is to say by their mobility relative to one another, with the result that, overall, either the ball screw can be constructed with somewhat smaller dimensions and/or the service life of the bearings increases significantly.

FIGS. 5 to 11 show a further brake device having a ball screw 24 according to the invention. In this arrangement, the invention may also be referred to as an actuating device for a parking brake.

Where components illustrated here correspond to those of the exemplary embodiment described above, the same reference numerals are used.

FIG. 5 shows, in section, a parking brake or immobilizing brake having the ball screw 24 according to the invention. Here, an axial bearing 25 is provided which is modified in relation to the preceding exemplary embodiment.

The ball screw 24 according to the invention with the axial bearing 25 is shown clearly in section in FIG. 6. A threaded nut 26 is mounted in a rolling fashion on a threaded spindle 28 in a known way by means of balls 27. The threaded spindle 28 has, outside its portion which interacts with the threaded nut 26, a radially stepped spindle portion 29 which is provided, on the axial end thereof, with a polygon 30. A gearing (not shown here) may be connected at the drive output side to said polygon 30.

FIG. 6 also shows that the threaded spindle 28 is guided with its spindle portion 29 through the axial bearing 25. The axial bearing 25 comprises a support disk 33 and an axial rolling bearing 38 in which rollers 39 are arranged between two bearing disks 40, 41. One bearing disk 40 bears against the support disk 33, and the other bearing disk 41 is supported against the housing-side portion 6.

FIG. 7 shows an enlarged detail of the ball screw 24 and of the axial bearing 25. The threaded spindle 28 is provided with a shoulder 31 at the transition to the radially recessed spindle portion 29. Said shoulder 31 has a bearing surface 32 which is convexly shaped with a radius of curvature. A support disk 33 of the axial bearing 25 is arranged on the threaded spindle 28 for conjoint rotation therewith, but such that it can perform a wobbling motion, via a toothing 34. The support disk 33 is provided, on its end side facing toward the first bearing surface 32, with a conical opening 35 which forms a second bearing surface 36.

The spindle axis S is indicated in FIG. 7. The radius of curvature R1 of the first bearing surface 32 intersects the spindle axis S. The two bearing surfaces 32, 36 make contact with one another along an annular contact path 37, the central point of which likewise lies on the spindle axis S. Said annular contact path 37 has a radius R2. It can be seen from FIG. 7 that the two radii R1 and R2 are arranged spaced apart from one another on the spindle axis S. The radius R1 is larger than the radius R2, wherein according to the invention, a quotient formed from the ratio of the radius R1 to the radius R2 assumes values between 1.4 and 1.6 inclusive. A circle drawn with the radius of curvature R1 lies in the plane of the page. A circle drawn with the radius of curvature R2 lies in a plane arranged perpendicular to the plane of the page.

FIG. 8 shows the situation in which, owing to an elastic deformation of the brake caliper 3 or of the housing-like portion 6, the support disk 33 is tilted relative to the threaded spindle in 28 by approximately 0.5°, wherein in the illustration, said tilt is illustrated on an exaggerated scale. Undesired loading of the axial bearing 25 with a bending moment is accordingly prevented. The support disk 33 is accordingly arranged on the threaded spindle 28 so as to be capable of performing a wobbling motion; said support disk can tilt about axes perpendicular to the spindle axis, and can transfer torques for the transmission of torques between support disk 33 and threaded spindle 28.

FIGS. 9 a, 9 b, 9 c show the support disk 33 in two views and in longitudinal section. In FIG. 9 b, pockets 42 for receiving lubricant are provided in the wall of the conical opening 35. A lubricating film is thus built up in the contact path 37, which lubricating film promotes free-moving tilting of the two bearing surfaces 32, 36.

FIG. 10 shows the ball screw according to the invention, with threaded nut 26 and support disk 33 illustrated in partially cut-away form. Here, it is possible to see a circumferential stop 43 for the threaded nut 26, which stop will be described in more detail below.

It can be seen from FIG. 10 that the support disk 33 is provided, on its end side facing toward the threaded nut 26, with an axial projection 44. Said axial projection 44 engages into a recess 45 of the threaded nut 26.

FIG. 11 clearly shows the recess 45, which extends in the circumferential direction over a relatively large circumferential segment. In one circumferential direction, the recess 45 is delimited by a tooth 46 which is integrally formed on the threaded nut 26 and which is directed radially inward. It can also be seen from FIG. 11 that the projection 44 is arranged in a stop position in which it abuts against a first stop surface 47 of the tooth 46.

In the axial direction, the recess 45 is delimited by a base 54 formed in one piece with the threaded nut 26. The recess is delimited in the radial direction by a circumferential wall 55 formed in one piece with the threaded nut 26.

Said stop 43 prevents the threaded nut 26 from being able to be clamped axially to the support disk 33. This is because, before end surfaces, which face toward one another, of the threaded nut 26 and of the support disk 33 come into contact with one another, the projection 44 abuts against the first stop surface 47 of the tooth 46.

The recess 45 extends over a circumferential angle of greater than 180°, such that the projection 44, upon a screw-type relative rotation with respect to the threaded nut 26, protrudes into said recess 45.

The circumferential stop 43 is designed such that, in the stop situation, a minimum spacing a is maintained between the threaded nut 26 and the support disk 33, such that at any rate axial clamping between the threaded nut 26 and threaded spindle 28 is prevented. FIG. 10 denotes the minimum spacing a which is provided between the two end surfaces, which face toward one another, of the threaded nut 26 and of the spindle disk 33.

In particular, it can be seen from FIG. 10 that the projection 44 and the first stop surface 47 overlap one another in the axial direction. Said axial overlap is on the one hand smaller than the overall axial extent of the axial projection 44, such that in any case, the abovementioned minimum spacing a is ensured. On the other hand, said axial overlap is larger than the axial extent of the projection 44 minus the axial minimum spacing a between the stop 43 and the threaded nut 26. Furthermore, the axial extent of the projection 44 is at most as large as the pitch of the ball screw in order to keep the bending moments acting on the projection 44 low at the instant of abutment against the first stop surface 47.

To prevent radial forces being generated owing to the abutment in the stop situation, in the stop position, a second stop surface 48 formed on the projection 44 and the associated first stop surface 47 of the tooth 46 are arranged in a common plane which contains the spindle axis.

The recess 45, which in the exemplary embodiment is formed on the end side of the threaded nut 26, extends in the circumferential direction over an angle formed from a quotient of the ratio of the abovementioned axial overlap to the pitch of the threaded spindle, multiplied by 360°, wherein to determine the angle, the axial overlap and the pitch of the threaded spindle are both designated using the same unit of length.

It can also be seen from FIG. 10 that in each case one optical marking 49, 50 is formed on the threaded nut 26 and on the support disk 33. Here, said markings 49, 50 are small depressions formed on the outer circumference. Said markings 49, 50 permit simple assembly of the ball screw 24, as will be explained in more detail below.

For correct functioning of the stop 43, the rotational position of the support disk 33 with respect to the threaded spindle 28 is of significance. For example, if, in the exemplary embodiment, the support disk 33 were arranged rotated counterclockwise about the threaded spindle by 90°, a situation could arise in which the threaded nut 26 and the support disk 33 abut against one another at the end side before the stop 43 has taken effect in the circumferential direction. Accordingly, correct rotational positioning of a stop part 51 with respect to the threaded spindle 28 is of significance. In the exemplary embodiment, the stop part 51 is formed by the support disk 33.

It can be seen from FIG. 11 that the toothing 34, already mentioned further above, between the support disk 33 and the spindle portion 29 of the threaded spindle 28 is provided for transmitting torques. Said toothing 34 allows the support disk 33 to be placed onto the spindle portion 29 in a plurality of rotational positions. Said toothing 34 is formed here by an external toothing 52 on the outer circumference of the spindle portion 29 and by an internal toothing 53 on the inner circumference of the support disk 33.

A tooth flank angle α of the external toothing 52 or of the internal toothing 53 is designed to be as small as possible, such that the steepest possible tooth flanks are formed. Steep tooth flanks facilitate the tilting mobility, described further above, of the support disk 33 with respect to the threaded spindle 28. The finer the toothing, the more rotational positions can be set.

For assembly of the ball screw 24, the threaded nut 26 may firstly be screwed onto the threaded spindle 28 until the threaded nut 26 has reached its intended stop position. The support disk 33 may then be placed onto the spindle portion 29 and rotated relative to the threaded spindle 28 and the threaded nut 26 until the two markings 49, 50 are arranged in alignment with one another. The support disk 33 may then be pushed axially further in the direction of the threaded nut 26, wherein the internal toothing 53 engages into the external toothing 52. It is also conceivable for two markings to be provided for example on the support disk 33, between which the marking 49 of the threaded nut 26 should be arranged. In this way, an angle is defined within which an admissible rotational position for the support disk 33 relative to the threaded spindle 28 is provided.

The assembly depicted here may take place in an automated fashion, wherein the markings 49, 50 can be detected by means of suitable measurement sensors. When said markings 49, 50 are in alignment with one another, by means of suitable control, the next assembly step can be triggered and the support disk 33 can be pushed with its internal toothing 53 onto the external toothing 52 of the spindle portion 29.

The ball screw may be formed without a ball return facility. This means that the balls are arranged in a non-endless ball channel and can merely roll back and forth between the ends of said ball channel. In the exemplary embodiment, a helical compression spring may be inserted into the ball channel, one end of which spring is supported against the tooth 46 and the other end of which spring is loaded against the final ball. During load-free ball screw operation, all the balls can be spring-loaded in the direction of the end of the ball channel under the action of a spring force of the helical compression spring. Alternatively, a ball screw may also be used which has, as is known, a ball return facility: the balls circulate in a continuous manner in endless ball channels. The ball channel is formed from a load portion, in which the balls roll under load on ball grooves of the threaded nut and of the threaded spindle, and a return portion, in which the balls are returned from an end to a beginning of the load portion. The return portion may be formed, in a known way, by a diverting pipe on the outer circumference of the threaded nut, or else by diverting pieces which are inserted in the wall of the threaded nut. Said diverting pieces connect an end of a common winding of the load portion to the beginning thereof.

In the exemplary embodiment, the threaded nut 26 with the recess 45 and the tooth 46 is formed from a case-hardened steel in the semi-hot state. Semi-hot forming is carried out in a temperature range from 750° C. to 950° C. For semi-hot forming, prefabricated untreated parts may be inductively heated and formed on partially multi-stage presses.

Here, the ball groove is produced in a cutting process by turning. Alternatively or in addition, the ball groove may also be produced by thread rolling. The finished threaded nut is subsequently case-hardened.

The support disk 33 is likewise produced in a non-cutting process, in particular in the semi-hot forming process. It can be seen in particular from FIG. 9 that the axial projection is approximately half pushed through. This means that material of the support disk 33 is formed out of the disk-shaped part, wherein the support disk 33 is provided, on its end side facing away from the projection, with a cavity.

LIST OF REFERENCE SYMBOLS

1 Brake device

2 Brake disk

3 Brake caliper

4 Brake pad

5 Brake pad

6 Housing-like portion

7 Ball screw

8 Threaded spindle

9 Balls

10 Threaded nut

11 Ball return element

12 Radial bearing

13 Axial bearing

14 Piston

15 Conical guide surface

16 Guide surface

17 Wall

18 First bearing disk

19 Second bearing disk

20 Needle rolling bodies

21 Axial projection

22 Second bearing surface

23 First bearing surface

24 Ball screw

25 Axial bearing

26 Threaded nut

27 Ball

28 Threaded spindle

29 Spindle portion

30 Polygon

31 Shoulder

32 First bearing surface

33 Support disk

34 Toothing

35 Conical opening

36 Second bearing surface

37 Contact path

38 Axial rolling bearing

39 Roller

40 Bearing disk

41 Bearing disk

42 Pocket

43 Stop

44 Projection

45 Recess

46 Tooth

47 First stop surface

48 Second stop surface

49 Marking

50 Marking

51 Stop part

52 External toothing

53 Internal toothing

54 Base

55 Circumferential wall

A Common axis

A₁ Axis of the brake caliper

A₂ Axis of the ball screw

A₃ Axis of the piston

R1 Radius of curvature of the first bearing surface

R2 Radius of the contact path

S Spindle axis 

1. A ball screw comprising a threaded spindle which is mounted in a rolling fashion by balls on a threaded nut and which is supported on an axial bearing, the threaded spindle is axially supported with a first bearing surface, which is convexly shaped with a radius of curvature (R1), on a conically designed second bearing surface, wherein the first bearing surface and the second bearing surface make contact with one another along a contact path, and a quotient formed from a ratio of the radius of curvature (R1) to the radius (R2) of the contact path has values of between 1.2 and 2.4 inclusive.
 2. The ball screw as claimed in claim 1, wherein the quotient is between 1.4 and 1.6 inclusive.
 3. The ball screw as claimed in claim 1, wherein a central point of the radius of curvature (R1) lies on a longitudinal axis (S) of the threaded spindle.
 4. The ball screw as claimed in claim 1, wherein at least one of the first and second bearing surfaces is provided with pockets for receiving lubricant.
 5. The ball screw as claimed in claim 1, wherein the axial bearing has a support disk which is connected to the threaded spindle in a positively locking manner in a rotational direction, the second bearing surface being formed on a side of said support disk which faces toward the threaded nut.
 6. The ball screw as claimed in claim 5, wherein the support disk is arranged to perform a wobbling motion on the threaded spindle.
 7. The ball screw as claimed in claim 5, wherein the support disk is provided, on a side facing away from the threaded nut, with an axial bearing surface which is formed directly on the support disk or on a bearing disk which adjoins the support disk. 